Organic phosphites are used in the stabilization of a wide variety of polymeric systems. Many different phosphites have been proposed for use either alone or in combination with other stabilizers. Such phosphites and their utilities are described in U.S. Pat. Nos. 4,371,647, 4,656,302, 4,705,879, 5,126,475, 5,141,975, and 5,438,086. The importance of organic phosphites as stabilizers has lead to the development of a variety of specialty organic phosphites that have enhanced effectiveness for stabilization.
Sterically hindered organic phosphites, and in particular diphosphites based upon pentaerythritol and containing alkyl, aryl, or alkyl-substituted aryl groups wherein the substitution is selected from the group consisting of t-butyl, t-amyl, t-hexyl, cyclohexyl, t-pentyl, and t-octyl, are especially desirable compounds due to their enhanced hydrolytic stability, ease of handling and compatibility with a wide variety of polymeric systems. The bis(2,4-di-tertbutylphenyl)pentaerythritol diphosphites are also especially preferred for their improved hydrolytic stability over other alkyl substituted phosphites as well as their enhanced compatibility with some polymeric resins, especially polyolefins.
The organic diphosphites are generally prepared using methods involving reactions between the appropriate hydroxy compounds and phosphorous trihalides, e.g., phosphorous trichloride. Such methods and other useful methods are described in U.S. Pat. Nos. 3,839,506, 4,116,926, 4,290,976, 4,440,696, and 4,492,661. The ease of substitution of the halides on the phosphorous trihalide decreases as each halide is replaced. For example, in the preparation of bis(aryl)pentaerithritol diphosphites, the pentaerithritol hydroxyls readily react with a phosphorous trihalide to yield a bis(disubstituted halo phosphite (i.e., an intermediate di-substituted diphosphorohalidite). The displacement of the third halo group is less than quantitative and is considerably slower in rate. Additionally, displacement of the third halo group by a sterically hindered phenol is even more difficult and requires elevated temperatures and/or use of a catalyst.
In order to increase the rate of reaction and the degree of completion for displacing the third halide with a sterically hindered moiety, various techniques have been generally utilized in the art. These techniques include: elevating the reaction mixture temperature and the use of hydrogen halide acceptors, e.g., amines. Such techniques are described in U.S. Pat. Nos. 3,281,506, 4,237,075, 4,312,818, 4,440,696, and 4,894,481.
Generally in the case of diphosphites derived from pentaerythritol, the procedures of the prior art result in undesirable product mixtures including caged structures wherein three of the hydroxyls on a single pentaerythritol have reacted with one phosphorous trihalide. Additionally, various polyphosphite compounds are also formed leading to low conversions to the desired product. The resulting phosphite mixture containing a halo-phosphite is extremely difficult to purify and the residual halo-phosphite can lead to acid impurities that affect the long term stability of the desired organic phosphite.
Various processes have been described in the prior art yet each suffers from some undesirable limitation. For example, U.S. Pat. No. 4,739,090 describes a process utilizing xylene as a solvent. The final product is isolated by filtration and the filtrate can be recycled. This process is deficient in resulting in at least about five percent or more impurities that require further crystallization to remove. This patent is silent on the form of the pentaerythritol utilized in the reaction.
U.S. Pat. No. 5,103,035 describes low temperature reaction conditions in chlorinated solvents. This process is undesirable due the difficulties in safely handling chlorinated solvents and a second solvent has to be utilized in order to bring the final product out of solution.
U.S. Pat. No. 5,438,086 describes a process for making diphosphites based upon pentaerythritol and 2,4-dicumylphenol wherein the dicumyl phenol is first reacted with phosphorous trichloride followed by allowing the reaction with the pentaerythritol. This process afforded only a 66% yield and acid numbers of 2 to 6, both of which are unacceptable.
It is therefore apparent that a need continues to exist for improved processes for the preparation of bis(dialkylphenyl)pentaerythritol diphosphites, and especially bis(2,4-di-tertbutylphenyl)pentaerythritol diphosphite, that overcome the aforementioned difficulties.